fix: use old nixpkgs for glibc only

.github/workflows/ci.yml

@@ -26,7 +26,7 @@ jobs:
           - name: Linux release
             os: ubuntu-latest
             release: true
-            shell: nix-shell --arg pkgs "import (fetchTarball \"channel:nixos-19.03\") {{}}" --argstr llvmPackages latest --run "bash -euxo pipefail {0}"
+            shell: nix-shell --arg glibc "(import (fetchTarball \"channel:nixos-19.03\") {{}}).glibc" --run "bash -euxo pipefail {0}"
             llvm-url: https://github.com/leanprover/lean-llvm/releases/download/13.0.0/lean-llvm-x86_64-linux-gnu.tar.zst
           - name: Linux
             os: ubuntu-latest

shell.nix

@@ -1,6 +1,6 @@
 let
   flakePkgs = (import ./default.nix).packages.${builtins.currentSystem};
-in { pkgs ? flakePkgs.nixpkgs, llvmPackages ? null }:
+in { pkgs ? flakePkgs.nixpkgs, llvmPackages ? null, glibc ? pkgs.glibc }:
 # use `shell` as default
 (attribs: attribs.shell // attribs) rec {
   shell = pkgs.mkShell.override {
@@ -14,7 +14,7 @@ in { pkgs ? flakePkgs.nixpkgs, llvmPackages ? null }:
     # more convenient `ctest` output
     CTEST_OUTPUT_ON_FAILURE = 1;
     GMP = pkgs.gmp.override { withStatic = true; };
-    GLIBC = pkgs.glibc;
+    GLIBC = glibc;
     shellHook = ''
       export LEAN_SRC_PATH="$PWD/src"
     '';